1. Field of the Invention:
This invention relates to an information reproducing apparatus for reproducing information signals recorded on a recording medium.
2. Description of the Related Art:
A still video apparatus shown in FIG. 1 which is an example of this kind of conventional apparatus for reproducing information signals has a reproducing system which makes use of a boosting transformer 5 to improve the signal-to-noise ratio of a reproduction signal if a thin film head or the like is employed as a magnetic head 1. A motor 3 is rotated at a constant speed under the control of a motor control circuit 4. The magnetic head 1 detects a signal from a magnetic recording medium 2 in the form of a disk which is rotated by the motor 3. This signal is boosted by the boosting transformer 5 so as to obtain an optimum carrier-to-noise (C/N) ratio, is amplified by a preamplifier 6, and thereafter undergoes amplitude correction in an equalizer 7, finally being demodulated into a baseband signal by an FM demodulator 6. The magnetic head 1 is capable of moving on a head moving rail 9 in the radial direction of the recording medium 2. The position of the magnetic head 1 is controlled by a head driving circuit 10. The magnetic head 1 is moved on the head moving rail 9 so that it reproduces signals from recording tracks formed on the magnetic recording medium 2.
As is well-known, a relative speed between the disk-like magnetic recording medium and the magnetic head becomes smaller as the head approaches the inner circumference of the recording medium. Correspondingly, the reproduction output is necessarily reduced. If the radius of an outermost recording track of a single magnetic recording medium is 20 mm; the radius of an innermost recording track is 15.1 mm; and the track pitch is 0.1 mm, this recording medium has 50 tracks available for recording. If the rotational speed of the recording medium is 3600 rpm, the speed of the first (outermost) track is 7.54 m/s, and the speed of the fiftieth (innermost) track is 5.69 m/s. The speed of the fiftieth track relative to the magnetic head is lower than that of the first track by 25%. In addition, if the recording frequency is 7 MHz, the recording wavelength is 1.1 .mu.m at the first track and is 0.8 .mu.m at the fiftieth track. The loss depending upon the recording wavelength is greater at the fiftieth track than it is at the first track. As a whole, there is a reduction of more than 25% in the output from the recording medium at the innermost track.
The boosting ratio of the boosting transformer 5 is determined so that the C/N is highest at the innermost track on the magnetic recording medium.
If a head reproduction output is E; the boosting ratio of the boosting transformer is n; the input-conversion noise level of the preamplifier is N.sub.A ; the level of head impedance noise is N.sub.h ; and the level of noise including tracing noise and modulation noise generating on the recording medium is Nm (N represents the level of noise certain frequency apart from the carrier, C represents the carrier), the C/N of a carrier having a single carrier frequency is represented by ##EQU1##
N.sub.A and N.sub.h are constant with respect to the position of the magnetic head on the recording medium in the radial direction while N.sub.m changes depending upon the radial position of the magnetic head such that it is larger at an outer circumferential position (where the wavelength is longer) than it is at an inner circumferential position (where the wavelength is shorter). Therefore, if N.sub.A and N.sub.h are on the same order, for example, they are several 0.1 nv/.sqroot.Hz, there are different relationships between n and C/N at the outer and inner circumferences of the recording area on the recording medium, as those shown in FIG. 2, in which the C/N begins to saturate at the outer circumference when n is n.sub.1 which is a value smaller than n.sub.2 which corresponds to the saturation at the inner circumference. As a result, the boosting ratio is generally set to a value as represented by n.sub.2 (relating to the change in C/N at the inner circumference).
However, if the boosting ratio of the boosting transformer is increased, the coupling coefficient tends to become smaller depending upon the form of the transformer or upon how the winding is formed, and the insertion loss thereby increases, resulting in a problem of a deterioration in the transmission characteristics.
To widen the band of recording signals on the recording medium, the resonance-point frequency of the recording head should be heightened as much as possible. It is therefore desired to reduce the inductance of the boosting transformer by minimizing the boosting ratio thereof.
Recently, many attempts have been made to achieve improvements in magnetic recording mediums or circuits for use in magnetic recording/reproduction apparatus for recording or reproducing information signals such as video signals in order to realize recording/reproduction of images with improved image qualities which cannot be achieved by conventional apparatus. In a system generally adopted for this purpose, the carrier frequency of a luminance signal recorded on a magnetic recording medium is increased in order to widen the video signal band and thereby realize image formation with a high degree of resolution.
In this case, many apparatus of this kind are designed, as in the case of a Super-VHS system for a video tape recorder, to provide a compatibility with ordinary conventional apparatus in such a manner that one recording/reproduction apparatus is constituted by two types of systems selectively used: one in which recording or reproduction is performed with the same frequency deviation as that of the conventional apparatus; and one in which the carrier frequency is heightened so as to expand the recording band for recording or reproduction of high-resolution video information.
FIG. 3 shows an example of this type of conventional magnetic recording/reproduction apparatus in which a video signal which has been input through a signal input terminal 11 is supplied via a switch S.sub.1 to one of a circuit 12 for processing high-resolution video information recording signals and a circuit 13 for processing low-resolution video information recording signals, is amplified by a recording amplifier 14 to a suitable amplitude, and is thereafter supplied as a recording current to a magnetic head 16 via a recording/reproduction change-over switch S.sub.2 and a boosting transformer 15 such as a rotary transformer, finally being recorded on a magnetic recording medium A in the form of a tape or disk.
When recording information recorded on the recording medium A in this manner is reproduced, the voltage of a signal detected by the magnetic head 16 is boosted to a suitable level by the boosting transformer 15, and the reproduction signal is thereafter supplied to a preamplifier 17 via a switch S.sub.2, and is supplied via a switch S.sub.3 to a circuit 18 for processing high-resolution video information reproduction signals if the signal contains high-resolution video information or to a circuit 19 for processing low-resolution video information reproduction signals if the signal contains conventional video information, the output from each circuit being output through an output terminal 20.
However, in the above example of the conventional apparatus, it is necessary to employ a recording medium improved in high-band characteristics in order to compensate for a reduction in the output of the high-band components of the video information signal in the high-image-quality mode in which the FM carrier frequency is increased. This is disadvantageous in terms of handling of the recording medium. It is also necessary to employ, as the recording/reproduction head, a narrow-gap head in order to improve the sensitivity in the high band of the video signal as much as possible. On the other hand, however, the use of a narrow-gap head entails a deterioration of the low-band reproduction sensitivity.